Thanks all for your observations. I may take a couple of litres out and top up with water. I have not done the maths but I think that would be about right.
I have not mixed different antifreeze as they are both Ford premium and I had the good sense to mix a sample first to ensure they were compatible. Ford apparently only make / sell one type of anti freeze now.
You can get a dinky little tool for optically measuring the antifreeze to make sure it's the right mix. Much better IMHO than the Specific Gravity types.
